Molecular diagnosis of anthelmintic resistance

Next-generation sequencing technologies for helminth diagnostics and surveillance in ruminants: shifting diagnostic barriers

Helminth infections in grazing ruminants are a major issue for livestock farming globally, but are unavoidable in outdoor grazing systems and must be effectively managed to avoid deleterious effects to animal health, and productivity. Next-generation sequencing (NGS) technologies are transforming our understanding of the genetic basis of anthelmintic resistance (AR) and epidemiological studies of ruminant gastrointestinal parasites. They also have the potential to not only help develop and validate molecular diagnostic tests but to be directly used in routine diagnostics integrating species-specific identification and AR into a single test. Here, we review how these developments have opened the pathway for the development of multi-AR and multispecies identification in a single test, with widespread implications for sustainable livestock farming for the future.

Evaluation of a combination of Citrus aurantium var. Dulcis essential oil and albendazole for the treatment of sheep gastrointestinal nematodes

Citrus fruits are consumed all over the world and their by-products are used for animal feed and essential oils production. This study aimed to evaluate the in vitro and in vivo activity of Citrus aurantium var. Dulcis essential oil (CaEO) combined with ABZ against benzimidazole resistant Haemonchus contortus. In vitro egg hatching assays (EHA) were performed using CaEO and ABZ to estimate the effective concentration to achieve 50% egg death (EC₅₀) values and calculate the test essential oil and drug combinations using a simplex-centroid mixture design. These concentrations were used for a second round of EHAs. Sixteen sheep were randomly allocated into two groups and treated with ABZ and the combination of CaEO and ABZ, and faecal egg count reduction tests were performed. In the first round of EHA, CaEO and ABZ showed EC₅₀ values of 0.57 and 0.0048 mg mL^-1, respectively. The H. contortus strain used in the study was shown to be highly benzimidazole resistant, with only 1.5% of parasites having susceptible ß-tubulin SNP genotypes. The ABZ reduced the shedding of nematode eggs by 78%, however, its combination with CaEO reduced faecal egg counts by only 9%. The present study is important to highlight the interferences of natural products in anthelmintic metabolism and consequently in drug efficacy.

An Inventory of Anthelmintic Plants across the Globe

A wide range of novelties and significant developments in the field of veterinary science to treat helminth parasites by using natural plant products have been assessed in recent years. To the best of our knowledge, to date, there has not been such a comprehensive review of 19 years of articles on the anthelmintic potential of plants against various types of helminths in different parts of the world. Therefore, the present study reviews the available information on a large number of medicinal plants and their pharmacological effects, which may facilitate the development of an effective management strategy against helminth parasites. An electronic search in four major databases (PubMed, Scopus, Web of Science, and Google Scholar) was performed for articles published between January 2003 and April 2022. Information about plant species, local name, family, distribution, plant tissue used, and target parasite species was tabulated. All relevant studies meeting the inclusion criteria were assessed, and 118 research articles were included. In total, 259 plant species were reviewed as a potential source of anthelmintic drugs. These plants can be used as a source of natural drugs to treat helminth infections in animals, and their use would potentially reduce economic losses and improve livestock production.

Advances in diagnosis of gastrointestinal nematodes in livestock and companion animals

Diagnosis of gastrointestinal nematodes in livestock and companion animals has been neglected for years and there has been an historical underinvestment in the development and improvement of diagnostic tools, undermining the undoubted utility of surveillance and control programmes. However, a new impetus by the scientific community and the quickening pace of technological innovations, are promoting a renaissance of interest in developing diagnostic capacity for nematode infections in veterinary parasitology. A cross-cutting priority for diagnostic tools is the development of pen-side tests and associated decision support tools that rapidly inform on the levels of infection and morbidity. This includes development of scalable, parasite detection using artificial intelligence for automated counting of parasitic elements and research towards establishing biomarkers using innovative molecular and proteomic methods. The aim of this review is to assess the state-of-the-art in the diagnosis of helminth infections in livestock and companion animals and presents the current advances of diagnostic methods for intestinal parasites harnessing (i) automated methods for copromicroscopy based on artificial intelligence, (ii) immunodiagnosis, and (iii) molecular- and proteome-based approaches. Regardless of the method used, multiple factors need to be considered before diagnostics test results can be interpreted in terms of control decisions. Guidelines on how to apply diagnostics and how to interpret test results in different animal species are increasingly requested and some were recently made available in veterinary parasitology for the different domestic species.

Anthelmintic resistance in ruminants: challenges and solutions

Anthelmintic resistance (AR) is a growing concern for effective parasite control in farmed ruminants globally. Combatting AR will require intensified and integrated research efforts in the development of innovative diagnostic tests to detect helminth infections and AR, sustainable anthelmintic treatment strategies and the development of complementary control approaches such as vaccination and plant-based control. It will also require a better understanding of socio-economic drivers of anthelmintic treatment decisions, in order to support a behavioural shift and develop targeted communication strategies that promote the uptake of evidence-based sustainable solutions. Here, we review the state-of-the-art in these different fields of research activity related to AR in helminths of livestock ruminants in Europe and beyond. We conclude that in the advent of new challenges and solutions emerging from continuing spread of AR and intensified research efforts, respectively, there is a strong need for transnational multi-actor initiatives. These should involve all key stakeholders to develop indicators of infection and sustainable control, set targets and promote good practices to achieve them.

Exploring the β-tubulin gene family in a benzimidazole-resistant Parascaris univalens population

Benzimidazole (BZ) drugs are frequently used to treat infections with the equine ascarid Parascaris univalens due to increasing resistance to macrocyclic lactones and pyrantel. Benzimidazole resistance is rare in ascarids in contrast to strongyle parasites where this resistance is widespread. In strongyles, single nucleotide polymorphisms (SNPs) at codons 167, 198 and 200 in a β-tubulin gene have been correlated to BZ resistance, but little is known about the β-tubulin genes and their possible involvement in BZ resistance in P. univalens and other ascarids. Previously two β-tubulin genes have been identified in P. univalens. In this study, we present five additional β-tubulin genes as well as the phylogenetic relationship of all seven genes to β-tubulins of other clade III and V nematodes. In addition, the efficacy of fenbendazole for treatment of P. univalens on a Swedish stud farm was studied in 2019 and 2020 using faecal egg count reduction test. Reductions varied from 73% to 88%, indicating the presence of a resistant P. univalens population on the farm. The emergence of BZ resistance emphasizes the need for development of molecular markers for rapid and more sensitive detection of resistant populations. We therefore investigated whether possible SNPs at positions 167, 198 or 200 in any of the β-tubulin genes could be used to distinguish between resistant and susceptible P. univalens populations. Amplicon sequencing covering the mutation sites 167, 198 and 200 in all seven β-tubulin genes revealed an absence of SNPs in both resistant and susceptible populations, suggesting that the mechanism behind BZ resistance in ascarids is different from that in strongyle nematodes and the search for a molecular marker for BZ resistance in P. univalens needs to continue.

•

First case of fenbendazole resistance in Parascaris univalens in Europe.

•

The P. univalens β-tubulin family contains seven genes.

•

P. univalens β-tubulin genes cluster with β-tubulins from other clade V nematodes.

•

No resistance associated SNPs were identified in P. univalens β-tubulin genes.

A loop-mediated isothermal amplification (LAMP) assay to identify isotype 1 β-tubulin locus SNPs in synthetic double-stranded Haemonchus contortus DNA

Development of sustainable gastrointestinal nematode (GIN) control strategies depends on the ability to identify the frequencies of drug-susceptible and resistant genotypes in GIN populations arising from management practices undertaken on individual farms. Resistance to BZ drugs in GINs has been shown to be conferred by the presence of defined SNPs in the isotype 1 β-tubulin locus. Loop-mediated isothermal amplification (LAMP) assays are amenable to use on a range of DNA templates and are potentially adaptable to use in practical, cost-effective, pen-side diagnostic platforms that are needed to detect anthelmintic resistance in the field. In this study, we designed primers and examined LAMP assays to detect each of the three major isotype 1 β-tubulin SNPs conferring genetic susceptibility to BZ drugs. We used artificial pools of synthetic DNA, containing different proportions of susceptible and resistant SNPs to determine reproducibility of the assays. We demonstrated the detection of each of the isotype 1 β-tubulin SNPs conferring susceptibility to BZ drugs using the optimal LAMP assay. Isotype 1 β-tubulin SNP typing was effective in detecting BZ susceptibility, but the accuracy was reduced in samples with less than 60 % susceptible DNA. Our results show the potential for LAMP SNP typing to detect genetic susceptibility or resistance to anthelmintic drugs in livestock GINs, and some of the limitations in our approach that will need to be overcome in order to evaluate this assay using field samples.

Reduced Efficacy of Fenbendazole and Pyrantel Pamoate Treatments against Intestinal Nematodes of Stud and Performance Horses

Nematodes are an important cause of disease and loss of performance in horses. Changes in the parasitic fauna of horses have occurred in the past few decades, making cyathostomins the major parasites in adult horses, while large strongyles have become less prevalent. Parascaris spp. remains the most important parasite infecting foals and weanlings. Anthelmintic resistance is highly prevalent in cyathostomins and Parascaris spp. worldwide and it must be factored into treatment decisions. To assess anthelmintic efficacy in Northern Italy, we sampled 215 horses from 17 sport and horse-breeding farms. Fecal egg count reduction tests (FECRT) were used to assess anthelmintic efficacy. Copromicroscopic analysis was performed using MiniFLOTAC before treatment with fenbendazole, pyrantel pamoate or ivermectin, and repeated 14 days post-treatment. Strongyle-type eggs were detected in 66.91% of horses (CI95% 61.40-73.79%), while Parascaris spp. was detected in 2.79% (CI95% 1.94-5.95%). Reduced efficacy against cyathostomins was observed for fenbendazole in 55.56% of the treated animals (CI95% 41.18-69.06%), and for pyrantel pamoate in 75% of animals (CI95% 30.06-95.44%). Ground-based actions must be set in place to promote the uptake of state-of-the-art worm control plans that will prevent clinical disease while minimizing the selection pressure of resistant parasites.

Haemonchus sp. in beef cattle in Brazil: species composition and frequency of benzimidazole resistance alleles

The aim of the present study was to investigate the occurrence of Haemonchus contortus and Haemonchus placei in beef cattle and the frequency of single nucleotide polymorphisms associated with benzimidazole (BZ)-resistance in Haemonchus spp. in Brazil. For such, fecal samples were collected from 61 beef cattle ranches in 11 Brazilian states. Third-stage larvae (L3) were produced for morphological species identification and DNA extraction. PCR was performed for the analysis of the isotype 1 β-tubulin gene and the products were sequenced to confirm the presence of H. placei and H. contortus. For each field population, pyrosequencing assays were performed to quantify the frequency of the F167Y, E198A and F200Y polymorphisms in the isotype-1 β-tubulin gene. The results of the morphometric analysis of 2345 larvae showed that H. placei was present on all ranches. The analysis of the isotype 1 β-tubulin gene confirmed 100% prevalence for H. placei and 23.7% for H. contortus. Pyrosequencing assays demonstrated single-nucleotide polymorphisms (SNPs) associated with BZ-resistance in all three codons (F167Y, E198A and F200Y) of the isotype 1 β-tubulin gene in H. placei field populations. Frequencies of resistance-associated alleles above background (≥ 15%) were found for at least one codon in 11.4% of the field isolates and maximum frequencies of 30, 21 and 29% were found for codons 167, 198 and 200, respectively, on individual ranches. This study confirms the presence of H. contortus in beef cattle in the major livestock farming states in Brazil and demonstrates that genotypes associated with BZ resistance are present in field populations of Haemonchus spp..

Molecular method for the semiquantitative identification of gastrointestinal nematodes in domestic ruminants

Standard diagnostic methods currently in use for the identification of helminth infections in ruminants are based on the morphological analysis of immature and adult stages of parasites. This paper describes a method for the semiquantitative identification of nematodes, mainly Trichostrongyloidea, at species-level resolution. The method is based on amplification and fragment analysis followed by minisequencing of the ITS-2 region (internal transcribed spacer 2) of the ribosomal DNA of parasite eggs or larvae. This method allows for the identification of seven genera (Chabertia, Cooperia, Haemonchus, Oesophagostomum, Ostertagia, Teladorsagia, and Trichostrongylus) and 12 species (Chabertia ovina, Cooperia curticei, Cooperia punctata, Cooperia oncophora/Cooperia surnabada, Haemonchus contortus, Haemonchus placei, Haemonchus longistipes, Oesophagostomum asperum, Oesophagostomum radiatum, Ostertagia ostertagi, Trichostrongylus axei, and Trichostrongylus colubriformis) of infectious nematodes of domestic ruminants. The concordance between the morphological and molecular analyses in the detection of genera ranged from 0.84 to 0.99, suggesting the proposed detection method is specific, semiquantitative, less laborious, and highly cost-efficient.

Development of amplicon sequencing for the analysis of benzimidazole resistance allele frequencies in field populations of gastrointestinal nematodes

Anthelmintic resistant gastrointestinal helminths have become a major cause of poor health in sheep and goats. Sensitive and specific molecular markers are needed to monitor the genotypic frequency of resistance in field parasite populations. Gastrointestinal nematode resistance to benzimidazole is caused by a mutation in one of three positions within the isotype 1 β-tubulin gene. In the absence of markers for resistance to the other broad spectrum anthelmintic classes, these provide a relevant study example. Determination of the prevalence of these single nucleotide polymorphisms in field nematode populations can be impractical using conventional molecular methods to examine individual parasites; which can be laborious and lack sensitivity in determining low levels of resistance in parasite populations. Here, we report the development of a novel method based on an Illumina MiSeq deep amplicon sequencing platform to sequence the isotype 1 β-tubulin locus of the small ruminant gastrointestinal nematode, Teladorsagia circumcincta, and determine the frequency of the benzimidazole resistance mutations. We validated the method by assessing sequence representation bias, comparing the results of Illumina MiSeq and pyrosequencing, and applying the method to populations containing known proportions of resistant and susceptible larvae. We applied the method to field samples collected from ewes and lambs on over a period of one year on three farms, each highlighting different aspects of sheep management and approaches to parasite control. The results show opportunities to build hypotheses with reference to selection pressures leading to differences in resistance allele frequencies between sampling dates, farms and ewes or lambs, and to consider the impact of their genetic fixation or otherwise. This study provides proof of concept of a practical, accurate, sensitive and scalable method to determine frequency of anthelmintic resistance mutations in gastrointestinal nematodes in field studies and as a management tool for livestock farmers.

Anthelmintic resistance and novel control options in equine gastrointestinal nematodes

Control of equine nematodes has relied on benzimidazoles (BZs), tetrahydropyrimidines and macrocyclic lactones. The intensive use of anthelmintics has led to the development of anthelmintic resistance (AR) in equine cyathostomins and Parascaris equorum. Field studies indicate that BZ and pyrantel resistance is widespread in cyathostomins and there are also increasing reports of resistance to macrocyclic lactones in cyathostomins and P. equorum. The unavailability of reliable laboratory-based techniques for detecting resistance further augments the problem of nematode control in horses. The only reliable test used in horses is the fecal egg count reduction test; therefore, more focus should be given to develop and validate improved methodologies for diagnosing AR at an early stage, as well as determining the mechanisms involved in resistance development. Therefore, equine industry and researchers should devise and implement new strategies for equine worm control, such as the use of bioactive pastures or novel feed additives, and control should increasingly incorporate alternative and evidence-based parasite control strategies to limit the development of AR. This review describes the history and prevalence of AR in equine nematodes, along with recent advances in developing resistance diagnostic tests and worm control strategies in horses, as well as giving some perspective on recent research into novel control strategies.

Genetic variants in dyf-7 validated by droplet digital PCR are not drivers for ivermectin resistance in Haemonchus contortus

Resistance to ivermectin (IVM) in the nematode Haemonchus contortus in small ruminants is an increasing problem throughout the world. Access to molecular diagnostics will allow early detection of IVM resistance, which in turn can limit the spread of resistant isolates. One candidate gene which has recently been suggested as a marker for IVM resistance is that for dye-filling protein (dyf-7). In this study, we critically investigated the suitability of A141G and G153T single nucleotide polymorphisms (SNP) of dyf-7 as a marker in larval cultures collected from sheep farms in Sweden, involving several isolates for which resistance status had been characterised by the faecal egg count reduction test (FECRT). Initially, we designed dyf-7 primers from a worldwide collection of adult Haemonchus contortus DNA. With the sequence data, we created a haplotype network. We then optimised and used the same sets of primers and probes in a droplet digital PCR (ddPCR) assay for precise quantification of dyf-7 allele frequencies in pre- and post-anthelmintic treatment faecal larval cultures. The fractional abundance (FA) of the mutant SNP was within the range 7.8 and 31%. However, the FA was generally stable in samples collected from the same farms, even though they were obtained on different occasions up to 25 months apart. There was also no indication that the level of IVM resistance as measured by the faecal egg count reduction test was higher on farms with high FA. Furthermore, by comparing FA in samples from the same farms pre- and post-IVM treatment, we found no evidence of a correlation between dyf-7 and level of IVM resistance. Based on these results, dyf-7 is not a suitable marker for field testing of IVM resistance in H. contortus.

Standardization and application of the tetraprimer ARMS-PCR technique for screening of the E198A SNP in the β-tubulin gene of hookworm populations in Brazil

The tetraprimer ARMS-PCR technique is efficient for SNP detection and can be used to search for polymorphisms associated with drug resistance. However, the establishment of this methodology is not always straightforward because of the constraints on primer design due to the restrictions of the polymorphic regions. Here, we describe the standardization of the tetraprimer ARMS-PCR methodology for the detection of a SNP at codon 198 of the Ancylostoma caninum β-tubulin gene. This SNP is associated with resistance to albendazole in various nematodes. The methodology was used to screen 327 individuals from 6 different locations. No mutation was found in any of the samples. This methodology will be useful for screening for the E198A SNP in the β-tubulin gene of canine hookworms in a broader population to determine whether this SNP is associated with benzimidazole resistance in this species. The method could also be adapted for the analysis of other SNPs in other nematode species.

Phenotypic assessment of the ovicidal activity of monepantel and monepantel sulfone on gastro-intestinal nematode eggs

The in vitro ovicidal activity of the amino acetonitrile derivative, monepantel (MPTL) and its active metabolite monepantel sulfone (MPTL-SO2) were assessed against a number of commercially important nematode species of ruminants, namely Teladorsagia circumcincta, Haemonchus contortus and Trichostrongylus axei. An egg hatch test (EHT) was used to make the assessment of both drug sensitive and drug resistant isolates. Both MPTL and MPTL-SO2 showed moderate ovicidal activity in vitro against all of the species examined, although species specific differences as measured by inhibitory concentration were observed. Analysis of the drug sensitive isolates showed H. contortus to be the most sensitive to both MPTL and MPTL-SO2 (ED50 1.7 and 2.7 μg/ml respectively) followed by T. circumcincta (ED50 2.1 and 2.7 μg/ml respectively) followed by T. axei (ED50 68.7 and 60.1 μg/ml respectively). Overall the EHT results would suggest no "global" in vitro discriminatory dose for detection of MPTL resistance is likely to be achievable, using the egg hatch test, due to large inherent variability observed between species. The test identified a dose dependent increase in MPTL and MPTL-SO2 sensitivity in two MPTL resistant T. circumcincta isolates and therefore offers to be a promising tool for the phenotypic characterisation of MPTL sensitivity, allowing exploration into the mechanisms involved in selection and development of MPTL resistance.

Absence of detectable benzimidazole-resistance associated alleles in Haemonchus placei in cattle in Nigeria revealed by pyrosequencing of β-tubulin isotype 1

Trichostrongyles are gastrointestinal parasites that occur globally and can cause subclinical to severe, sometimes life-threatening, infections in ruminants, particularly young animals. Benzimidazoles (BZ) are commonly used for the treatment of gastrointestinal parasites in ruminants. Increasing spread of worm populations with anthelmintics resistance has been reported and is considered a consequence of highly frequent and longstanding use of anthelmintics. To obtain initial information regarding the occurrence of putatively BZ-resistant Nigerian Haemonchus populations, screening based on the molecular analysis of BZ-resistance-associated β-tubulin isotype 1 gene sequence polymorphisms was undertaken. Genomic DNA was isolated from pooled adult Haemonchus sp. from 35 animals from each of the six states of southwestern Nigeria. Sequencing of internal transcribed spacer 2 (ITS-2) and external transcribed spacer (ETS) regions was used to determine the Haemonchus species. Pyrosequencing assays were used for detection of single-nucleotide polymorphisms (SNPs) in the β-tubulin isotype 1 genes of the worms at codons 200 and 167 (TTC/TAC) or 198 (GAA/GCA). Exclusively, Haemonchus placei was detected and allele frequencies obtained at all three positions showed no evidence for the presence of resistance-related alleles. For Lagos State, pools of 10 worms from 30 different animals were analyzed separately for the codon 200 SNP, successfully excluding the presence of resistance-associated SNPs in very low frequencies. These positive findings, showing absence of elevated frequencies of BZ-resistance-associated β-tubulin alleles, have considerable significance since it suggests that farmers can still rely on the efficacy of this important drug class when used for controlling trichostrongyle infections in cattle in Nigeria.

Resistência anti-helmíntica em nematoides gastrintestinais de pequenos ruminantes: avanços e limitações para seu diagnóstico

A seleção e a crescente disseminação de nematoides resistentes aos anti-helmínticos mais comumente utilizados, benzimidazóis (BZs), imidazotiazóis e lactonas macrocíclicas (LMs), constituem um sério entrave na produção de pequenos ruminantes em todo o mundo. O uso de métodos eficientes e sensíveis para a detecção e o monitoramento da resistência anti-helmíntica no campo torna-se urgente, especialmente para os grupos de BZs e LMs, devido aos constantes relatos de resistência. A obtenção de um diagnóstico preciso e precoce da resistência é extremamente importante para auxiliar a tomada de decisão em programas de controle parasitário, com o objetivo de preservar a vida útil dos produtos e limitar o desenvolvimento da resistência nas populações de nematoides. Os testes in vivo e, mais recentemente, os testes in vitro têm sido desenvolvidos para a detecção de nematoides resistentes aos principais grupos de anti-helmínticos. No entanto, a disponibilidade de testes in vitro validados e o seu uso prático ainda são muito limitados. Embora o teste de redução na contagem de ovos nas fezes (TRCOF, in vivo - indireto) seja o principal método de escolha para a detecção de resistência no campo, vem recebendo críticas quanto à validade dos resultados, e passa por significativas modificações. Além disso, o desenvolvimento de técnicas moleculares a partir de alterações genômicas gerou avanços consideráveis nessa área de investigação, com o uso de mutações nos códons 167, 198 e 200 do gene da β-tubulina como principais SNPs (polimorfismos de nucleotídeo único; do inglês Single Nucleotide Polymorphisms) associados à resistência aos BZs. A presente revisão tem o objetivo de discutir os métodos de diagnóstico disponíveis para a detecção de resistência anti-helmíntica em nematoides de pequenos ruminantes, destacando progressos e obstáculos para seu uso na rotina laboratorial e no campo.

Advances in the diagnosis of key gastrointestinal nematode infections of livestock, with an emphasis on small ruminants

Parasitic nematodes (roundworms) of livestock have major economic impact globally. In spite of the diseases caused by these nematodes and some advances in the design of new therapeutic agents (anthelmintics) and attempts to develop vaccines against some of them, there has been limited progress in the establishment of practical diagnostic techniques. The specific and sensitive diagnosis of gastrointestinal nematode infections of livestock underpins effective disease control, which is highly relevant now that anthelmintic resistance (AR) is a major problem. Traditional diagnostic techniques have major constraints, in terms of sensitivity and specificity. The purpose of this article is to provide a brief background on gastrointestinal nematodes (Strongylida) of livestock and their control; to summarize conventional methods used for the diagnosis and discuss their constraints; to review key molecular-diagnostic methods and recent progress in the development of advanced amplification-based and sequencing technologies, and their implications for epidemiological investigations and the control of parasitic diseases.

Impact of gastrointestinal parasitic nematodes of sheep, and the role of advanced molecular tools for exploring epidemiology and drug resistance - an Australian perspective

Parasitic nematodes (roundworms) of small ruminants and other livestock have major economic impacts worldwide. Despite the impact of the diseases caused by these nematodes and the discovery of new therapeutic agents (anthelmintics), there has been relatively limited progress in the development of practical molecular tools to study the epidemiology of these nematodes. Specific diagnosis underpins parasite control, and the detection and monitoring of anthelmintic resistance in livestock parasites, presently a major concern around the world. The purpose of the present article is to provide a concise account of the biology and knowledge of the epidemiology of the gastrointestinal nematodes (order Strongylida), from an Australian perspective, and to emphasize the importance of utilizing advanced molecular tools for the specific diagnosis of nematode infections for refined investigations of parasite epidemiology and drug resistance detection in combination with conventional methods. It also gives a perspective on the possibility of harnessing genetic, genomic and bioinformatic technologies to better understand parasites and control parasitic diseases.

Interactions of anthelmintic drugs in Caenorhabditis elegans neuro-muscular ion channel mutants

Due to the increasing development of anthelmintic resistance in nematodes worldwide, it is important to search for anthelmintic compounds with new modes of action and also to investigate the possibility to combine compounds with possible synergistic effects. There might also be the chance to take advantage of the fact that nematode populations which have developed resistance against one anthelmintic class might respond hypersusceptibly to another drug class. The aim of this study was to investigate responses of Caenorhabditis elegans populations with mutations in neuro-muscular ion channels to different anthelmintic classes. Furthermore, potential synergistic effects between two anthelmintic compounds from different classes, i.e. emodepside and tribendimidine, were studied. Although there was neither a synergistic nor an antagonistic effect between emodepside and tribendimidine, other types of interactions could be identified. The C. elegans GABAA-receptor (GABAA-R) unc-49 mutants, showing decreased emodepside susceptibility, were more susceptible to tribendimidine than wild-type C. elegans. In contrast, the reverse phenomenon - hypersusceptibility to emodepside in tribendimidine resistant acetylcholine-receptor (AChR) loss of function mutants - was not observed. Moreover, the slo-1 mutant strain (completely emodepside resistant) also showed hypersusceptibility to piperazine. Interestingly, neither the GABAA-R unc-49 mutants nor the AChR mutants showed decreased susceptibility against piperazine, although there were some studies that indicated an involvement of GABAA-R or AChR in the piperazine mode of action. In conclusion, the present study provides evidence suggesting that interactions between commercially available anthelmintic drugs with different modes of action might be a relatively common phenomenon but this has to be carefully worked out for each anthelmintic and each anthelmintic drug combination. Moreover, results obtained in C. elegans will have to be confirmed using parasitic nematodes in the future.

Next-generation molecular-diagnostic tools for gastrointestinal nematodes of livestock, with an emphasis on small ruminants: a turning point?

Parasitic nematodes of livestock have major economic impact worldwide. Despite the diseases caused by these nematodes, some advances towards the development of new therapeutic agents and attempts to develop effective vaccines against some of them, there has been limited progress in the development of practical diagnostic methods. The specific and sensitive diagnosis of parasitic nematode infections of livestock underpins effective disease control, which is now particularly important given the problems associated with anthelmintic resistance in parasite populations. Traditional diagnostic methods have major limitations, in terms of sensitivity and specificity. This chapter provides an account of the significance of parasitic nematodes (order Strongylida), reviews conventional diagnostic techniques that are presently used routinely and describes advances in polymerase chain reaction (PCR)-based methods for the specific diagnosis of nematode infections. A particular emphasis is placed on the recent development of a robotic PCR-based platform for high-throughput diagnosis, and its significance and implications for epidemiological investigations and for use in control programmes.

Decreased emodepside sensitivity in unc-49 γ-aminobutyric acid (GABA)-receptor-deficient Caenorhabditis elegans

Emodepside, a semi-synthetic derivative of PF1022A, belongs to a new class of anthelmintic drugs, the cyclooctadepsipeptides, and shows good efficacy against macrocyclic lactone-, levamisole- or benzimidazole-resistant nematode populations. Although putative receptors for emodepside have already been discovered, its mode of action is still not fully understood. The involvement of the γ-aminobutyric acid (GABA)-receptor on the PF1022A mode of action has previously been postulated. Therefore, a possible role of the GABA-receptor, unc-49, in the mode of action of emodepside was investigated using two different Caenorhabditis elegans in vitro assays, a motility assay and a development assay. It was found that there is a clearly reduced sensitivity against emodepside of strains carrying a GABA-receptor, unc-49, loss of function mutation compared with N2 wild type C. elegans. To transfer these results from the model system to parasitic nematodes, the Toxocara canis unc-49B cDNA sequence was identified and used in a rescue experiment. The emodepside-susceptible phenotype could be fully rescued by injection of the T. canis unc-49B cDNA sequence. We believe that this is the first functional rescue of a C. elegans mutant strain with a gene from a clade III parasitic nematode. These findings, together with the earlier data on GABA-receptor binding of PF1022A, suggest that the GABA(A)-receptor UNC-49 is associated with the emodepside mode of action. However, the only partially resistant phenotype of the loss of function mutants indicates that other pathways play a more significant role.

The soluble proteome phenotypes of ivermectin resistant and ivermectin susceptible Haemonchus contortus females compared

Anthelmintics in the absence of vaccines have underpinned a parasite control strategy for over 50 years. However, the continued development of anthelmintic resistance (AR) threatens this control. Measuring early AR is difficult as there many routes that resistance can arise from within multi-nematode populations operating complex metabolism capabilities coupled to different drug management pressures. There is an urgent need to identify and measure early resistance in the field situation. Proteomic profiling of expressed soluble proteins offers a new approach to reveal a drug resistant phenotype within a complex protein pattern. The hypothesis under test was that established differences in drug response phenotypes between nematode isolates can also be measured in their comparative proteomes. As a case study, proteomic differences were measured between an ivermectin resistant and susceptible adult female Haemonchus contortus. Adult H. contortus females were extracted from the abomasa of six lambs. The nematodes had been maintained in the lambs as monospecific isolates of either ivermectin susceptible or ivermectin resistant worms. Comparative analysis of the soluble proteome was completed along with immuno-proteomic analysis using pooled infection sera from the lambs. Following image analysis, spots of interest were excised and analysed by peptide mass fingerprinting and the proteins putatively identified using BLAST. Overall, a relative increase in the expression of proteins involved in the detoxification metabolic area was observed in the resistant isolate. In addition, Western blotting analysis also revealed differences in immuno-reactivity profiles between resistant and susceptible isolates. It can be concluded from this study that proteomic differences can be detectable between ivermectin susceptible and a resistant isolates of H. contortus, which could be further explored using other isolates to confirm if proteomic based fingerprinting offers molecular phenotyping or a new panel of resistance biomarkers.

A molecular diagnostic tool to replace larval culture in conventional faecal egg count reduction testing in sheep

The accurate diagnosis of parasitic nematode infections in livestock (including sheep and goats) is central to their effective control and the detection of the anthelmintic resistance. Traditionally, the faecal egg count reduction test (FECRT), combined with the technique of larval culture (LC), has been used widely to assess drug-susceptibility/resistance in strongylid nematodes. However, this approach suffers from a lack of specificity, sensitivity and reliability, and is time-consuming and costly to conduct. Here, we critically assessed a specific PCR assay to support FECRT, in a well-controlled experiment on sheep with naturally acquired strongylid infections known to be resistant to benzimidazoles. We showed that the PCR results were in close agreement with those of total worm count (TWC), but not of LC. Importantly, albendazole resistance detected by PCR-coupled FECRT was unequivocally linked to Teladorsagia circumcincta and, to lesser extent, Trichostrongylus colubriformis, a result that was not achievable by LC. The key findings from this study demonstrate that our PCR-coupled FECRT approach has major merit for supporting anthelmintic resistance in nematode populations. The findings also show clearly that our PCR assay can be used as an alternative to LC, and is more time-efficient and less laborious, which has important practical implications for the effective management and control strongylid nematodes of sheep.

Anthelmintic resistance in sheep in Europe: a selected review

Intensive use of anthelmintics to control gastrointestinal nematodes selects for anthelmintic resistance, which has become an important issue in many European countries. Presence of nematode strains resistant to benzimidazoles, imidazothiazoles and/or macrocyclic lactones has been repeatedly reported, particularly for the three most important genera, Haemonchus, Teladorsagia and Trichostrongylus. Additionally, multiple drug-resistant populations of these parasites have also been detected. Examples are given for the situation in European countries with differing climatic conditions and management systems of small ruminants. The widespread emergence of multi-resistant nematodes proves that the past intensive suppressive chemical control strategies may not be a successful approach any longer. Experience from the up to now development of anthelmintic resistance suggests that modern control schemes should not rely on sole use of anthelmintics, but employ other, more complex and sustainable recipes, combing chemical, environmental and immunological control.

Relative level of thiabendazole resistance associated with the E198A and F200Y SNPs in larvae of a multi-drug resistant isolate of Haemonchus contortus

While the F200Y SNP in the beta-tubulin gene is most commonly associated with benzimidazole resistance in trichostrongylid nematodes, other SNPs as well as drug efflux pathways have been implicated in the resistance. The relative contributions of all these mechanisms are not understood sufficiently to allow expected drug efficacy to be inferred from molecular data. As a component of developing better means to interpret molecular resistance tests, the present study utilised a drug resistant Haemonchus contortus isolate which possesses two of the principal benzimidazole resistance SNPs (E198A and F200Y) in order to assess the relative degree of resistance conferred by the two SNPs. We exposed larvae to a range of thiabendazole concentrations in in vitro development assays, and collected the surviving L3 larvae at each drug concentration to establish sub-populations showing increasing levels of resistance. We then sequenced the isotype 1 beta-tubulin gene in pooled larval samples, and measured allele frequencies at the two SNP positions. The frequency of the resistance allele at the 198 position increased as the thiabendazole concentration increased, while the frequency of the resistance allele at the 200 position decreased. Genotyping of individual larvae showed that the highest drug concentration was associated with the removal of all genotypes except for homozygous resistance at the 198 position alongside homozygous susceptible at the 200 position. This indicates that, at least for larval life stages, the E198A SNP is able to confer higher levels of resistance to benzimidazole drugs than the F200Y SNP, and that the homozygosity at 198 in the highly resistant individuals is mutually exclusive with heterozygosity or resistant homozygosity at the 200 position. This study illustrates the need to understand the relative contributions of different resistance mechanisms in order to maximise the degree to which molecular tests are able to inform on drug resistance phenotype.

Evolution of acaricide resistance: phenotypic and genotypic changes in field populations of Rhipicephalus (Boophilus) microplus in response to pyrethroid selection pressure

There have been few, if any, studies of arthropod field populations quantifying the kinetics of evolution of phenotypic and genotypic resistance to chemicals in response to the presence or absence of selection pressure. A prospective intervention study was undertaken over 2 years in Mexico to measure changes in resistance phenotype and genotype in the presence or absence of pyrethroid selection pressure on field populations of Rhipicephalus (Boophilus) microplus ticks on 11 farms with varying degrees of pyrethroid resistance. The resistance phenotype was evaluated by bioassay in a larval packet test expressed as the resistance factor (RF) derived from probit analysis of dose mortality regressions, and resistance genotype by an allele-specific PCR (AS-PCR) to determine the frequency of a sodium channel mutation (F1550I) associated with pyrethroid resistance. To validate the AS-PCR, a Pyrosequencing™ method was developed to detect the F1550I mutation. There was good concordance with the genotypes identified by both Pyrosequencing™ and AS-PCR (Kappa: 0.85). On five farms cypermethrin (CY) was exclusively used at intervals and on six farms amitraz was used. On two of the five CY-treated farms, the experiment was prematurely terminated due to unacceptably high levels of tick resistance. For all five farms, after 8-24 months of continued selection pressure with CY, the RF had increased 2-125-fold. The frequency of the resistance allele increased on all five farms from a starting range of 5-46% to a range of 66-95% after 8-24 months. On six farms treated with amitraz neither the RF nor the frequency of the resistance allele changed. A clear correlation between the phenotype and genotype was found in three of four treated farms confirming that the F1550I mutation is a major cause of synthetic pyrethroid resistance in Mexico. These results show that the pyrethroid resistance trait is stable (> 2 years) and that resistance is acquired much faster than it is lost. Hence, alternation of pyrethroid acaricide with other chemicals is likely to lead to the stepwise acquisition of synthetic pyrethroid resistance but not additional prolongation of its efficacious lifespan.

P-glycoprotein interfering agents potentiate ivermectin susceptibility in ivermectin sensitive and resistant isolates of Teladorsagia circumcincta and Haemonchus contortus

P-glycoprotein (P-gp) homologues, belonging to the ATP Binding Cassette (ABC) transporter family, are thought to play an important role in the resistance of gastro-intestinal nematode parasites against macrocyclic lactones. The aim of this study was to investigate the influence of various P-gp interfering compounds on the efficacy of ivermectin (IVM) in sensitive and resistant nematode isolates. The feeding of IVM resistant and sensitive Teladorsagia circumcincta and Haemonchus contortus first-stage larvae (L1) was assessed using a range of IVM concentrations (0.08-40 nm) with or without P-gp inhibitors: valspodar, verapamil, quercetin, ketoconazole and pluronic P85. The P-gp inhibitors were selected on the basis of their ability to interfere with P-gp transport activity in an epithelial cell line over-expressing murine P-gp. In the presence of P-gp interfering agents, the in vitro susceptibility to IVM of both sensitive and resistant isolates of T. circumcincta and H. contortus was increased. These results show that compounds interfering with P-gp transport activity could enhance IVM efficacy in sensitive isolates, and also restore IVM sensitivity in resistant nematodes. These results support the view that ABC transporters can play an important role in resistance to IVM, at least in the free-living stages of these economically important gastro-intestinal nematodes.

A combined microscopic-molecular method for the diagnosis of strongylid infections in sheep

We evaluated a combined microscopic-molecular approach for the diagnosis of key strongylid infections in sheep using panels of well-defined control and test samples. The method established is based on the separation of nematode eggs from faecal samples using a salt flotation procedure, the extraction and column-purification of genomic DNA, followed by real-time PCR and melting-curve analysis. Specific and semi-quantitative amplification from (a minimum of 0.1-2.0pg) genomic DNA of Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus spp., Cooperia oncophora, Oesophagostomum columbianum, Oesophagostomum venulosum or Chabertia ovina is achieved using a specific, forward oligonucleotide primer located in the second internal transcribed spacer (ITS-2) of nuclear ribosomal DNA (rDNA) together with a conserved reverse primer in the large subunit of rDNA. Using a panel of well-defined genomic DNA samples from eggs from sheep monospecifically infected with H. contortus or Te. circumcincta, there was a correlation between cycle threshold (Ct) values in the PCR and numbers of egg per gram of faeces, thus allowing the semi-quantitation of parasite DNA in faeces. The findings of the present study indicate that a microscopic-molecular approach provides a useful tool for diagnosis, for epidemiological and ecological surveys as well as for integration into parasite monitoring, drug resistance (i.e. 'egg count reduction') testing or control programmes, particularly following semi- or full-automation.

Resistencia a los antihelmínticos en nematodos gastrointestinales de bovinos en municipios de Cundinamarca y Boyacá

En 36 fincas lecheras de la Sabana de Bogotá, los Valles de Ubaté y Chiquinquirá y la Región del Tequendama (Cundinamarca, Colombia) se determinó la resistencia a los antihelmínticos en nematodos gastrointestinales de bovinos mediante la prueba in vivo de la reducción del conteo de huevos (RCH). En cada finca se seleccionaron 40 bovinos de tres a 12 meses de edad, los cuales se distribuyeron en cuatro grupos de 10 animales: 1) control (no tratado), 2) albendazol 25% (dosis: 5 mg/kg peso vivo), 3) ivermectina 1% (0,2 mg/kg) y 4) levamisol 18,8% (1 mg/kg). En cada grupo se identificaron mediante coprocultivos seis géneros de parásitos siendo Cooperia spp. el predominante. La resistencia se declaró cuando el porcentaje de RCH fue menor de 95% y el límite inferior del intervalo de confianza (95%), menor a 90. El 25% de las fincas presentó resistencia: en 17% se detectó resistencia al albendazol y en 8% a la ivermectina; para estos dos antihelmínticos, Cooperia spp. fue el nematodo gastrointestinal involucrado. El promedio de reducción de huevos por gramo de heces (hpg), luego de los tratamientos con ivermectina, albendazol y levamisol, fue de 97,89% ± 6% (66% - 100%), 95,9% ± 9,3% (51% - 100%) y 99,4% ± 0,92% (97,6% - 100%), respectivamente. No se detectó resistencia al levamisol. Los factores de riesgo asociados con la resistencia fueron: vermifugación de bovinos adultos, dosificación no acorde con el peso de los animales y uso de un mismo principio activo por un tiempo mayor de cuatro años.   

Understanding triclabendazole resistance

Triclabendazole (TCBZ) has been the drug of choice to treat liver fluke infections in livestock for >20 years, due to its high activity against both adult and juvenile flukes. More recently, it has been used successfully to treat human cases of fascioliasis. Resistance to TCBZ first appeared in the field in Australia in the mid-1990s. Since then, resistance has been reported from a number of countries throughout Europe: Ireland, Scotland, Wales, Spain and The Netherlands. The heavy reliance on a single drug puts treatment strategies for fascioliasis at risk. Should resistance develop further, the prospect is an alarming one. This review will present an overview of progress in understanding the mechanism of resistance to TCBZ, examining possible changes in the target molecule, in drug influx/efflux mechanisms and in the metabolism of TCBZ by the fluke. The review will also consider ways to deal with resistance, covering drug-oriented options such as: the use of alternative drugs, drug combinations and the search for new compounds.

Anthelmintic resistance and alternative control methods

Anthelmintic resistance has been a problem almost from the first use of the drugs introduced during the past 50 years. Evaluation of anthelmintics on a specific farm is essential to determine which drugs may be used in controlling parasite numbers. Treatment of livestock during the season in which parasite transmission is unfavorable lessens pasture contamination. Targeting treatment places less pressure on susceptible worms, diluting the resistant parasite population. Pasture management and using safe pastures for animals at highest risk lower pasture exposure. Selection for individual animals resistant to the effects of parasites lessens the need for use of anthelmintics. Alternatives to anthelmintics include cryptic antigen vaccines, copper wires, and biologic compounds.

Real-time PCR for quantifying Haemonchus contortus eggs and potential limiting factors

The purpose of this study was to evaluate the practicality of using real-time PCR for quantifying feces-derived trichostrongyle eggs. Haemonchus contortus eggs were used to evaluate fecal contaminants, time after egg embryonation, and the presence of competing and non-competing DNAs as factors that might interfere with generating reproducible results during simplex and multiplex quantitative real-time PCR (QPCR). Real-time PCR results showed linear quantifiable amplification with DNA from five to 75 eggs. However, threshold cycle (C (T)) values obtained by amplification of DNA from egg numbers between 75 and 1,000 did not differ significantly. Inhibitors of QPCR were effectively removed during DNA extraction as exemplified by the absence of any improvement in C (T) values with bovine serum albumin or phytase treatments. Changes from egg embryonation could only be detected during the first 6 h. Noncompetitive DNA did not appear to impact amplification; however, in a multiplex reaction a competing trichostrongyle such as Cooperia oncophora can hinder amplification of H. contortus DNA, when present at tenfold greater amounts. This study demonstrates the usefulness of QPCR for amplification and quantification of trichostrongyle eggs, and identifies potential limitations, which may be addressed through multiplex assays or the addition of a standard: exogenous DNA target.

Innovative restriction site created PCR-RFLP for detection of benzimidazole resistance in Teladorsagia circumcincta

Benzimidazole compounds, especially albendazole, are the most commonly used anthelmintics for deworming of small ruminants in Iran. It is believed that the therapeutic effects of the benzimidazoles (BZs) come through their binding capacity to the beta-tubulin isotype 1. Substitution of phenylalanine to tyrosine at position 200 of this polypeptide confers resistance to BZs. Several investigators developed different biological- and molecular-based techniques to demonstrate the occurrence of resistance in helminthes against BZs. To address the determination of resistance at position 200 of beta-tubulin isotype 1, we developed an innovative restriction site created polymerase chain reaction-restriction fragment length polymorphism, in which nucleotide A at the position of 637 upstream flanked by the first two coding sequences of the phenylalanine (TT) triplet was substituted through the nucleotide G. The introduced modification in forward primer (UTvet MF-primer) leads to the creation of restriction site (AACGTT) for PSP1. Therefore, in the case of normal allele only, PSP1 can cut the corresponding PCR product. In the first step, the genomic DNA was isolated from each single Teladorsagia circumcincta collected either from the abomasa of untreated (n = 35) or of 5 mg/kg BW 2.5% albendazole suspension-treated (n = 40) sheep. It was amplified with the primer pair, creating PCR product of 403 bp in length. In the second step, the PCR product was extracted from agarose gel and amplified with the modified forward primer (UTvet MF-primer) and the same reverse primer as in step 1, creating a PCR product of 222 bp. The PCR product was then cut with PSP1 to obtain in the case of normal allele two DNA products (183 and 39 bp). Eight of the 35 worms collected from the untreated sheep were BZ(SS) homozygotes, and the rest (27) were BZ(RS) heterozygotes. In our preliminary experiment, we could not find a BZ(RR) homozygote form within the examined samples. Five out of 40 worms collected from the albendazole-treated sheep were BZ(RR) homozygotes, whereas the rest (35) were BZ(RS) heterozygotes. No BZ(SS) homozygote form was detected within this group.

Understanding anthelmintic resistance: The need for genomics and genetics

Anthelmintic resistance is a major problem for the control of many parasitic nematode species and has become a major constraint to livestock production in many parts of the world. In spite of its increasing importance, there is still a poor understanding of the molecular and genetic basis of resistance. It is unclear which mutations contribute most to the resistance phenotype and how resistance alleles arise, are selected and spread in parasite populations. The main strategy used to identify mutations responsible for anthelmintic resistance has been to undertake experimental studies on candidate genes. These genes have been chosen predominantly on the basis of our knowledge of drug mode-of-action and the identification of mutations that can confer resistance in model organisms. The application of these approaches to the analysis of benzimidazole and ivermectin resistance is reviewed and the reasons for their relative success or failure are discussed. The inherent limitation of candidate gene studies is that they rely on very specific and narrow assumptions about the likely identity of resistance-associated genes. In contrast, forward genetic and functional genomic approaches do not make such assumptions, as illustrated by the successful application of these techniques in the study of insecticide resistance. Although there is an urgent need to apply these powerful approaches to anthelmintic resistance research, the basic methodologies and resources are still lacking. However, these are now being developed for the trichostrongylid nematode Haemonchus contortus and the current progress and research priorities in this area are discussed.